Liner hanging apparatus

ABSTRACT

Apparatus for setting a liner pipe in a well casing including a hydraulically actuated liner hanger having a piston-cylinder mechanism with seal means comprising two steel piston rings and a tetrafluoroethylene ring therebetween and having longitudinal protective ribs between toothed anchoring slips and extending radially at least as far as the slip teeth and including a float shoe having primary and secondary surge-type check valves to facilitate the creation of a pressure surge or pulse and actuation of the liner hanger.

United States Patent Brown v [4 1 June 26, 1973 [54] LINER HANGING APPARATUS 867,735 10/1907 Lewin 166/137 2,916,092 12/1959 Burns 166/208 [751' odma 3,455,381 7/1969 Page, Jr. 166/208 [73] Assignee: Brown Well Service & Supply Company, Odessa, T x, Primary Examiner-James A. Leppink Film} p 8 1971 l lt ornqy-TomArnold, Jack C. Goldstem et a1.

[21] Appl. No.: 178,596 [57] ABSTRACT Apparatus for setting a liner pipe in a well casing in- 52 US. Cl 166/208, 166/212, 166/241 cludins hydraulically actuated liner hanger having a 511 1111. c1 E2111 43/10 Piston-cylinder mechanism with Seal means comprising [58] F1011! 61 Search 166/208, 212, 216, two Piston rings and tetrafluoroethylene g 1 /217 118 13 139 140 137 241 therebetween and having longitudinal protective ribs between toothed anchoring slips and extending radially 55 g Cited at least as far as the slip teeth and including a float shoe UNITED STATES PATENTS having primary and secondary surge-type check valves 2 345 888 4/1944 8 n 6 8 to facilitate the creation of a pressure surge or pulse 2:328:840 9/1943 O ':c;! y III IIII 166/208 and actuation ofthe liner hanger 3,460,615 8/1969 Watkins 166/208 15 Claims, 6 Drawing Figures PAIENTEDJms ms SEE-1N2 FIG] PAIENIEDJUIZB ma Edd FIG. 5

LINER HANGING APPARATUS BACKGROUND OF THE INVENTION This invention relates generally to apparatus for use in completing a well, and more particularly, to apparatus for use in hanging a liner pipe from a well casing.

It is common practice in the course of drilling or completing a well, such as an oil or gas well, to insert and perhaps cement in place strings of pipe of diminishing diametric extent into the well bore. After the largest diameter pipe, called a casing, is set and cemented in place, the next smaller pipe, called a liner pipe, having an outside diameter smaller than the inside diameter of the casing, is lowered into the well bore telescopically through the casing by means of a releasable connector or setting tool at the lower end of a tubing string or drill pipe, so that it depends into the well bore below the lower end of the casing. As is well known in the art, the liner pipe may be floated into the well bore as it is lowered thereinto by means of a float shoe attached to the lower extremity of the liner pipe. After the liner pipe is lowered to its desired position, the liner pipe is suspended from the casing by means of a liner hanger at the upper end of the liner pipe, which liner hanger is activated into gripping engagement with the casing. Thereafter, the liner pipe may or may not be cemented into place; and the tubing string and setting tool are removed from the well. Of course, subsequent smaller diameter liner pipes may thereafter be suspended from the next larger liner pipe by the same general method. Thus, it should be understood that, although reference is herein continually made to suspending a liner pipe from a casing, this invention is applicable to suspending any smaller diameter pipe from a larger diameter pipe previously set in a well bore.

In order to overcome a number of problems previously recognized in the art, liner hangers that are hydraulically actuated, such as disclosed in U. S. Pat. No. 3,570,599 issued to Wilson et al on Mar. 16, 1971,

have come into widespread use. In certain aspects, thisinvention may be considered an improvement in the liner hanger disclosed in that patent and other hydraulically actuated liner hangers.

Most hydraulically actuated liner hangers utilize some kind of a piston-cylinder mechanism wherein a fluid-tight seal is effected by an O-ring, which is an annular rubber member usually having a round cross section. That piston-cylinder mechanism will ordinarily release anchoring slips so that they may be expanded into gripping engagement to set the liner pipe. Applicant has discovered, however, that, as sand or other solid particles become wedged between the O-ring and the adjacent piston or cylinder wall, the piston-cylinder mechanism will frequently jam, thereby rendering the liner hanger totally inoperative since the slips cannot be expanded into gripping engagement with the casing.

Another problem that has heretofore existed in the art is how to obtain within the liner pipe a pressure surge or pulse of sufficient magnitude to actuate a hydraulic liner hanger, without using a great deal of complex apparatus that may obstruct production flow and that may be difficult to remove from the liner pipe. One solution to that problem is disclosed in my copending application Ser. No. 107,784, filed Jan. 19, i971, entitled "Well Pipe String Valves."

One further problem that has heretofore existed in the art is that the external teeth of the anchoring slips are frequently damaged by sharp contact with the interior of the casing as the liner and liner hanger are lowered into place.

SUMMARY OF THE INVENTION In accordance with one embodiment of this invention, there are provided between the slips of a liner hanger protective ribs extending axially at least over the logitudinal position of the slip teeth and radially outward at least as far over the slip teeth at the corresponding longitudinal when the slips are in the unactuated position. Also, the fluid seal means in the pistoncylinder mechanism of the hydraulic actuation apparatus may include a metal piston ring or a tetrafluoroethylene ring or two metal piston rings with a tetrafluoroethylene ring therebetween. Still further, a float shoe may be provided with a primary surge-type check valve and, perhaps, a secondary surge-type check valve; and those valves or perhaps the conventional check valves should include a tubular resilient member as the biasing means.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily appreciated as the same becomes better understood by reference to the following'detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a vertical section view of a portion of a well bore having a casing therein, and including an elevation view of a liner hanger and float shoe according to the invention as used with other conventional members;

FIG. 2 is a vertical elevation view, partly in section of a portion of a liner hanger according to the present invention;

FIGS. 3 and 4 are vertical elevation sectional views of a portion of a liner hanger, particularly the hydraulically operated latch release mechanism therefor, according to the present invention;

FIG. 5 is a vertical elevation view, partly in section, of a float shoe according to the present invention; and

FIG. 6 is a vertical elevation view, partly in section, of a variation of part of the apparatus shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings wherein like reference characters designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, there is shown a well casing 10 previously set in a well bore 12, all of which is conventional in the art. A conventional liner pipe 14, having a float shoe 16 secured to its lower extremity, is secured to the lower extremity of a liner hanger 18 by a conventional coupling mechanism 20, all of which is secured to the lower extremity of a conventional tubing string or drill pipe 22 by means of a conventional releasable connector or setting tool 24. Liner hanger 18 is shown in FIG. 1 in its unactuated condition, although it is in a position where it would be actuated and set, since liner pipe 14 is shown in a producing formation 26. It should be understood that, after liner hanger 18 has been actuated and set in well casing 10 to suspend liner pipe 14 in the proper location in well bore 12, tubing string 22 is appropriately operated to actuate releasable connector 24 so that the tubing string may be withdrawn from the well, leaving the liner pipe and liner hanger in place for production. 7

As more clearly shown in FIG. 2, liner hanger 14 includes a tubular hanger body 28 which generally supports all of the parts of the liner hanger including a downwardly tapering conically shaped slip expander 30 secured to the upper extremity of the hanger body by any suitable means, such as threaded connection 32 and weld bead 34. It should be apparent that the slip expander and hanger body may be secured to one another by other techniques or might even be integrally formed as a single member.

Circumscribing hanger body 28 below slip expander 30 is a slidably mounted tubular slip cage 36 to which one end of a plurality of bow springs or spring stabilizers 38 are secured by any convenient means, such as screws 40. The bow springs are shaped to normally assume a position larger than the inside diameter of the well casing so that, when lowered down into the casing, the bow springs will frictionally engage the interior surface of the casing. Thus, when the latch release mechanism is actuated as will be described and tubing string 22 with attached liner hanger 18 is further lowered, the frictional engagement of bow springs 38 on the interior surface of casing will cause slip cage 36 to remain stationary, that is, move upward relative to hanger body 28.

A plurality of externally toothed anchoring slips 42 are secured to the upper extremity of slip cage 36 via strap springs 44 secured therebetween by any convenient means, such as screws 46. Strap springs 44 are so shaped to normally resiliently bias slips 42 radially inward. Thus, when slip cage 36 is in its lowermost position relative to hanger body 28 as shown in FIG. 4, slips 42 are radially retracted against the exterior surface of hanger body 28 below slip expander 30. However, it should be apparent that when slip cage 36 moves upward relative to hanger body 28 as has been described, slips 42 will ride up on the slip expander and will be radially outwardly displaced thereby so that the externally toothed surfaces of the slips will be forced out against and into gripping engagement with the interior surface of well casing 10.

Although the structure thus far described herein is somewhat different from the corresponding structure shown in U. S. Pat. No. 3,570,599 and other prior art devices, it should be apparent that the important aspect of the present invention now to be described is applicable thereto as well as to various other embodiments which include externally toothed anchoring slips, whether the slips are actuated hydraulically or otherwise. A plurality of longitudinal protective ribs 48 are positioned between adjacent pairs of slips by any suitable technique, such as being welded to hanger body 28 and/or slip expander 30. Each of the ribs must extend radially outward at least as far as and preferably beyond the slip teeth at the corresponding longitudinal position when the slips are radially retracted, as shown in FIG. 2. Although the exterior contour of the radially retracted slips is generally upwardly tapered, the ribs Although the ribs must extend radially outward at least as far as and preferably beyond the slip teeth, the ribs must be sufficiently radially narrow to pass through the well casing; thus, the ribs would not extend radially outward as far as the outermost point of the bow springs, for example. Although the ribs could be somewhat shorter than is shown in the preferred embodiment, the ribs must also extend axially at least as far as and pref erably beyond the slip teeth when the slips are radially retracted. Preferably, there is at least one rib between each adjacent pair of slips. Thus, the ribs, extending radially and axially at least as far as and preferably beyond the slip teeth when the slips are retracted, effectively protect the slip teeth from damage by unnecessary contact with the well casing as the liner hanger is lowered thereinto and therethrough. Those protective ribs constitute an important aspect of the present invention.

As shown in FIG. 2, positioned at the lower extremity of slip cage 36 are a plurality of elongated latch springs 50 having a hook or detent 52 adapted normally to engage a catch ring or circular band 54 fixedly secured to hanger body 28 by any convenient means, such as by welding or the like. Although latch springs 50 are illustrated herein as being formed integrally with slip cage 36, it should be apparent that the latch springs might be separate from but secured to the slip cage by any convenient means, such as by bolts, rivets, welding,'or

the like.

Referring now to FIG. 3, there is shown a hydraulically operated latch release mechanism including a tu bular chamber body 56 circumscribing and sealingly secured to hanger body 28 by suitable means, such as threaded connection 58. An annular piston 60 is axially slidingly disposed between hanger body 28 and the upper counterbored extremity of chamber body 56 and a plurality of seal means 62 are provided therebetween, thereby forming an annular expansible hydraulic pressure chamber 64 in fluid communication with the interior of liner hanger 18, as well as liner pipe 14 and tubing string 22, via a plurality of radial ports 66 provided in the hanger body at circumferentially spaced points. The upper end of piston 60 is provided with a curved cam surface 68 so shaped that, when the piston is forced upward, hooks 52 on latch springs 50 will be forced outward to clear catch ring 54, thereby disengaging slip cage 36. Of course, piston 60 may be caused to move upward by admission of fluid under pressure are not similarly tapered according to the preferred embodiment since the greater radial extension at the upper portion is not detrimental. Although the outermost surface of each rib is generally planar and parallel to the axis of the hanger body according to the preferred embodiment, it should be understood that the ribs need not be flat and could be upwardly tapered.

from the interior of liner pipe 14, liner hanger 18, and tubing string 22 into expansible hydraulic pressure chamber 64 through ports 66.

After hooks 52 have been disengaged from catch ring 54 as just described, tubing string 22 is lowered further and the frictional engagement of bow springs 38 on the interior surface of well casing 10 causes the bow springs, slip cage 36, and slips 42 to move upward relative to hanger body 28 and slip expander 30, thereby causing the slips to grip the casing wall. If it is necessary to move the liner pipe thereafter, liner hanger 18 may be released by pulling up on the tubing string so that the slip expander is pulled upward relative to the slips and away therefrom. The bow springs will then cause latch springs 50 to move downward relative to catch ring 54, and, owing to the correspondingly tapered surfaces on hooks 52 and the catch ring, the hooks will slide downward over the catch ring until they again engage the catch ring. If the slips do not readily break free of the well casing, the weight of the tubing string may be used to free the slips by lowering the tubing string slightly.

Although the hydraulically operated latch release mechanism thus far described herein is somewhat different from either embodiment thereof shown in U. S. Pat. No. 3,570,599 and other prior art devices, it should be apparent that the important aspect of the present invention now to be described is applicable thereto as well as to various other embodiments of hydraulically operated latch release mechanisms. As shown in greater detail in FIG. 4, a plurality of annular seal means 62 are provided between hanger body 28 and piston 60 and between chamber body 56 and piston 60 in recesses formed in the piston. Instead of round cross-section rubber O-rings conventionally employed to provide a fluid-tight seal between the piston means and chamber means, seal means 62 includes two metal piston rings 70, preferably steel, and one tetrafluoroethylene ring 72 positioned between the two piston rings. With the two metal piston rings abutting against each side of the tetrafluoroethylene ring, the seal means has a generally rectangular cross-sectional shape, the elongate flat sides of which resist rolling movement and the intrusion of sand or other particles between the seal means and the adjacent chamber means wall. Although seal means constructed in accordance with the preferred embodiment have been found to be highly reliable and effective despite the presence of sand or other particles in the fluid inside the hydraulic pressure chamber or in the annular space between the liner hanger and the well casing,'it should be understood that other variations of seal means may be suitable in overcoming the difficulties experienced with the use of O-rings. For example, one or more metal piston rings could, perhaps, be used without the tetrafluoroethylene ring, or the tetrafluoroethylene ring could be used with one or no metal piston rings. It should also be apparent that other materials having physical and chemical properties generally corresponding to steel and tetrafluoroethylene may be used in lieu of those presently preferred materials. Of course, seal means having other cross-sectional shapes or made of other materials may be suitable in overcoming the difficulties posed by the presence of sand or other particles, now that the detrimental effects of sand or other particles on O-rings have been discovered and disclosed by applicant.

Referring now to FIG. 5, there is shown in greater detail float shoe 16 which is especially suitable for use with hydraulically actuated liner hangers, such as liner hanger 18. As is customary in the art, float shoe 16 is secured to liner pipe 14 by any convenient means, such as threaded connection 74. Similarly, the float shoe has a plurality of primary outlet ports 78 at the lower extremity thereof and includes one or more conventional one-way or back flow check valves 76 presenting substantially no'resistance to downward fluid flow through the liner pipe, the float shoe and the ports but preventing substantially all fluid flow in the opposite direction. Check valves of the type just described have heretofore been used in float shoes in order to float liner pipes into wells in a manner that is old in the art; however, those valves, offering substantially no resistance to downward fluid flow, had no capacity for permitting the creation of a pressure surge or pulse thereabove of sufficient magnitude to actuate a hydraulic liner hanger.

Positioned in shoe 16 below valves 76 but above primary outlet ports 78 is a primary surge-type check valve 80 and a secondary surge-type check valve 82. Primary surge-type check valve 80 includes a valve seat means 84, a valve 86 having an elongate valve stem 88 secured thereto, all of which is preferably made of brass, Micarta, or the like, and a resilient biasing means 90, such as a tubular resilient member 92 preferably made of neoprene rubber or the like axially compressed between a pair of end plates 94 by a pressure adjusting nut 96 tightly screwed down on threaded end 98 of the valve stem, urging the valve toward its seated or closed position. It should be apparent that, by tightly screwing nut 98 down onto valve stem 88, tubular member 92 becomes highly axially compressed, thereby enabling primary check valve 80 to remain closed unless a differential pressure above some predetermined magnitude sufficient to preclude premature actuation, such as 700 p.s.i., exists across the valve. Such a pressure surge may be easily effected from conventional pumps at the well site.

Although the aforementioned surge effect may be produced in either of valves 76 instead of valve 80 by using a stiffer coil spring tightly compressed by an adjusting nut, the use of a solid tubular resilient member has an advantage thereover in that an added degree of reliability is achieved since particles cannot be trapped between the spring coils. Moreover, it has also been found desirable when using a surge-type check valve to provide a plurality of small diameter bleed ports 100 in float shoe 16 providing fluid communication between the interior of the float shoe above the primary surgetype check valve and the annular space between the liner pipe and the interior surface of the well bore. Bleed ports 100 are believed to be useful in preventing premature actuation of a hydraulically actuated liner hanger upon raising of the liner pipe with fluid inside thereof since those ports may enable the pressures to neutralize.

An alternative embodiment of resilient biasing means for primary surge-type check valve 80 is shown in FIG. 6 and includes a plurality of coil springs 102 provided in longitudinal bores 104 in tubular resilient member 92 made of neoprene rubber or the like and axially compressed between end plates 94-by pressure adjusting nut 96 screwed down on threaded end 98 of valve stem 88. Although coil springs are used in the embodiment shown in FIG. 6, tubular resilient member 92 effectively precludes particles from wedging between adjacent coils of the coil springs.

Referring again to FIG. 5, there is shown a secondary or auxiliary surge-type check valve 82 which may or may not be used when a primary surge-type check valve is used. When it is desired to use a secondary surge-type check valve, valve seat means 84 of primary surge-type check valve 80 is slidingly mounted. in float shoe l6, upward movement being limited by an internal stop ring 106 formed or welded in float shoe 16 which is also provided with a plurality of large diameter radial ports 108. When valve seat means 84 is in its uppermost or closed position, a fluid-tight seal is effected between the valve-seat means and the float shoe above ports 108 by a plurality of annular seal means 110, which may be O-rings, metal piston rings, tetrafluoroethylene rings, or a combination thereof as previously described in relation to the liner hanger. Valve seat means 84 is resiliently biased toward its uppermost or retainer ring 116 and member 112 and a slight radial clearance 120 between member 112 and float shoe l6,

3. The apparatus of claim 1 wherein said ribs extend longitudinally above the lower extremity of said slip expander.

4. The apparatus of claim 1 wherein the radially outermost surface of each rib is generally planar,

5. The apparatus of claim ,1 wherein at least one rib is positioned between every adjacent pair of slips.

accommodation may be made for radial expansion as a result of axial compression of member 112.

The secondary surge-type check valve provides a convenient safety feature if, for any reason, the primary surge-type check does not open upon the application of a p.s.i. differential pressure. By increasing the pressure still further to, perhaps, 1000 psi. differential pressure, valve seat means 84 and the remainder of the primary surge-type check valve would be displaced downward against the resilient force provided by axially compressed member 112 below ports 108 so that fluid flow from the inside of the liner pipe may be effected therethrough. i i g Obviously, numerous modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent of theUnited States is:

1. In a liner hanger adapted to support a depending liner pipe from a well casing disposed in a well bore, said liner hanger comprising a tubular hanger body adapted to be secured at its lower extremity to the upper extremity of the liner P P I a'downwardly tapering slip expander mounted on said hanger body,

a plurality of normally radially retracted circumferentially spaced externally toothed anchoring slips slidably mounted on said hanger body between a first position below said slip expander whereat said slips are radially retracted and a second position in overlying engagementwith said slip expander whereat said slips are outwardly radially expanded into gripping engagement with the interior surface of the well casing, and hydraulic actuating means for axially upwardly displacing said slips relative to said hanger body from said first position to said second position, the improvement comprising protective ribs mounted on at least one of said hanger body and said slip expander, said ribs extending axially at least over the longitudinal position of the slip teeth when said slips are in said first position and radially outward at least as far as the slip teeth at the corresponding longitudinal position when said slips are retracted in said first position, thereby reducing the unnecessary contact of the slip teeth with the well casing as the liner hanger and depending liner pipe are lowered into place.

2. The apparatus of claim 1 wherein said ribs are welded to at least one of said hanger body and said slip expander.

6. The apparatus of claim 6 wherein said actuating means comprises chamber, means forming a portion of an expansible hydraulic pressure chamber;

piston means slidably mounted relative to said cham-,

ber means and forming another portion of said expansible hydraulic pressure chamber; and seal means positioned between said piston means and said chamber means for effecting a fluid-tight seal therebetween, i

the cross section of said seal means comprising two generally straight, parallel sides engaging said piston means and said chamber means, respectively,

thereby resisting rolling movement but being readily susceptible to sliding movement as said piston means slides relative to said chamber means. i

.7. The apparatus of claim 6 wherein said seal means comprises a metal piston ring.

8. The apparatus of claim 6 wherein said seal means comprises a tetrafluoroethylene ring.

9. The apparatus of claim 6 wherein said seal means comprises a plurality of rings.

10. The apparatus of claim 6 wherein said seal means comprises two metal piston rings and one tetrafluoroethylene ring positioned between said metal piston rings.

11. In a liner hanger adapted to supp'orta depending liner pipe from a well casing disposed in a well bore, said liner hanger comprising a tubular hanger body adapted to be secured at its lower extremity to the upper extremity of the liner P P r a downwardly tapering slip expander mounted on said hanger body,

a plurality of normally radially retracted circumferentially spaced externally toothed anchoring slips slidably mounted on said hanger body between a first position below said slip expander whereat said slips are radially retracted and i a second position in overlying engagement with said slip expander whereat said slips are outwardly radially expanded into gripping engagement with the interior surface of the well casing,

and

hydraulic actuating means for axially upwardly displacing said slips relative to said, hanger body from said first position to said second position, said actuating means comprising chamber means forming a portion of an expansible hydraulic pressure chamber;

piston means slidably mounted relative to said chamber means and forming another portion of said exthereby resisting rolling movement but being readily susceptible to sliding movement as said piston means slides relative to said chamber means. 12. The apparatus of claim 11 wherein said seal means comprises a metal piston ring.

13. The apparatus of claim 11 wherein said seal means comprises a tetrafluoroethylene ring.

14. The apparatus of claim 11 wherein said seal ton rings. 

1. In a liner hanger adapted to support a depending liner pipe from a well casing disposed in a well bore, said liner hanger comprising a tubular hanger body adapted to be secured at its lower extremity to the upper extremity of the liner pipe, a downwardly tapering slip expander mounted on said hanger body, a plurality of normally radially retracted circumferentially spaced externally toothed anchoring slips slidably mounted on said hanger body between a first position below said slip expander whereat said slips are radially retracted and a second position in overlying engagement with said slip expander whereat said slips are outwardly radially expanded into gripping engagement with the interior surface of the well casing, and hydraulic actuating means for axially upwardly displacing said slips relative to said hanger body from said first position to said second position, the improvement comprising protective ribs mounted on at least one of said hanger body and said slip expander, said ribs extending axially at least over the longitudinal position of the slip teeth when said slips are in said first position and radially outward at least as far as the slip teeth at the corresponding longitudinal position when said slips are retracted in said first position, thereby reducing the unnecessary contact of the slip teeth with the well casing as the liner hanger and depending liner pipe are lowered into place.
 2. The apparatus of claim 1 wherein said ribs are welded to at least one of said hanger body and said slip expander.
 3. The apparatus of claim 1 wherein said ribs extend longitudinally above the lower extremity of said slip expander.
 4. The apparatus of claim 1 wherein the radially outermost surface of each rib is generally planar.
 5. The apparatus of claim 1 wherein at least one rib is positioned between every adjacent pair of slips.
 6. The apparatus of claim 6 wherein said actuating means comprises chamber means forming a portion of an expansible hydraulic pressure chamber; piston means slidably mounted relative to said chamber means and forming another portion of said expansible hydraulic pressure chamber; and seal means positioned between said piston means and said chamber means for effecting a fluid-tight seal therebetween, the cross section of said seal means comprising two generally straight, parallel sides engaging said piston means and said chamber means, respectively, thereby resisting rolling movement but being readily susceptible to sliding movement as said piston means slides relative to said chamber means.
 7. The apparatus of claim 6 wherein said seal means comprises a metal piston ring.
 8. The apparatus of claim 6 wherein said seal means comprises a tetrafluoroethylene ring.
 9. The apparatus of claim 6 wherein said seal means comprises a plurality of rings.
 10. The apparatus of claim 6 wherein said seal means comprises two metal piston rings and one tetrafluoroethylene ring positioned between said metal piston rings.
 11. In a liner hanger adapted to support a depending liner pipe from a well casing disposed in a well bore, said liner hanger comprising a tubular hanger body adapted to be secured at its lower extremity to the upper extremity of the liner pipe, a downwardly tapering slip expander mounted on said hanger body, a plurality of normally radially retracted circumferentially spaced externally toothed anchoring slips slidably mounted on said hanger body between a first position below said slip expander whereat said slips are radially retracted and a second position in overlying engagement with said slip expander whereat said slips are outwardly radially expanded into gripping engagement with the interior surface of the well casing, and hydraulic actuating means for axially upwardly displacing said slips relative to said hanger body from said first position to said second position, said actuating means comprising chamber means forming a portion of an expansible hydraulic pressure chamber; piston means slidably mounted relative to said chamber means and forming another portion of said expansible hydraulic pressure chamber; and seal means positioned between said piston means and said chamber means for effecting a fluid-tight seal therebetween, the cross section of said seal means comprising two generally straight, paralLel sides engaging said piston means and said chamber means, respectively, thereby resisting rolling movement but being readily susceptible to sliding movement as said piston means slides relative to said chamber means.
 12. The apparatus of claim 11 wherein said seal means comprises a metal piston ring.
 13. The apparatus of claim 11 wherein said seal means comprises a tetrafluoroethylene ring.
 14. The apparatus of claim 11 wherein said seal means comprises a plurality of rings.
 15. The apparatus of claim 11 wherein said seal means comprises two metal piston rings and one tetrafluoroethylene ring positioned between said metal piston rings. 